Commitments and Contingent Liabilities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Mar. 31, 2011
Commitments and Contingencies Disclosure [Line Items]
Operating Leases, Future Minimum Payments Due	$ 215.0
Operating Leases, Future Minimum Payments Due, Current	28.8
Operating Leases, Future Minimum Payments, Due in Two Years	28.4
Operating Leases, Future Minimum Payments, Due in Three Years	22.9
Operating Leases, Future Minimum Payments, Due in Four Years	18.7
Operating Leases, Future Minimum Payments, Due in Five Years	13.9
Operating Leases, Future Minimum Payments, Due Thereafter	102.3
Operating Leases, Rent Expense, Minimum Rentals	36.1	29.3	30.1
Commitment to fund partnership Equity Investments	65.4
Commitments to fund special purpose entity	0.5
Commitment to fund private placement fixed maturity securities	35.0
Unfunded commitment to fund Investment Partnerships	$ 160.6
States involved in unclaimed property examination				26
Insurers requested to cross-check benefits	160